Old Persian is one of two directly attested Old Iranian languages (the other being Avestan) and is the ancestor of Middle Persian (the language of the Sasanian Empire). Like other Old Iranian languages, it was known to its native speakers as ariya (Iranian). Old Persian is close to both Avestan and Vedic Sanskrit, and all three languages are highly inflected.

Old Persian appears primarily in the inscriptions, clay tablets and seals of the Achaemenid era (c. 600 BCE to 300 BCE). Examples of Old Persian have been found in what is now Iran, Romania (Gherla), Armenia, Bahrain, Iraq, Turkey and Egypt, with the most important attestation by far being the contents of the Behistun Inscription (dated to 522 BCE).

In 2007, research into the vast Persepolis Administrative Archives at the Oriental Institute at the University of Chicago unearthed Old Persian tablets, which suggest Old Persian was a written language in use for practical recording and not only for royal display.

As a written language, Old Persian is attested in royal Achaemenid inscriptions. It is an Iranian language and as such a member of the Indo-Iranian branch of the Indo-European language family. The oldest known text written in Old Persian is from the Behistun Inscriptions. Old Persian is one of the oldest Indo-European languages which are attested in original texts.

The oldest date of use of Old Persian as a spoken language is not precisely known. According to certain historical assumptions about the early history and origin of ancient Persians in Southwestern Iran (where Achaemenids hailed from), Old Persian was originally spoken by a tribe called Parsuwash, who arrived in the Iranian Plateau early in the 1st millennium BCE and finally migrated down into the area of present-day Fārs province. Their language, Old Persian, became the official language of the Achaemenid kings. Assyrian records, which in fact appear to provide the earliest evidence for ancient Iranian (Persian and Median) presence on the Iranian Plateau, give a good chronology but only an approximate geographical indication of what seem to be ancient Persians. In these records of the 9th century BCE, Parsuwash (along with Matai, presumably Medians) are first mentioned in the area of Lake Urmia in the records of Shalmaneser III. The exact identity of the Parsuwash is not known for certain, but from a linguistic viewpoint the word matches Old Persian pārsa itself coming directly from the older word *pārćwa. Also, as Old Persian contains many words from another extinct Iranian language, Median, according to P. O. Skjærvø it is probable that Old Persian had already been spoken before the formation of the Achaemenid Empire and was spoken during most of the first half of the first millennium BCE.

Old Persian belongs to the Iranian language family, a branch of the Indo-Iranian language family, itself within the large family of Indo-European languages. The common ancestors of Indo-Iranians came from Central Asia sometime in the first half of the 2nd millennium BCE. The extinct and unattested Median language is another Old Iranian language related to Old Persian; both are classified as Western Iranian languages, and many Median names appear in Old Persian texts. The group of Old Iranian languages was presumably large; however, knowledge of it is restricted mainly to Old Persian, Avestan, and Median. The first two are the only languages in that group to have left written original texts, while Median is known mostly from loanwords in Old Persian.

By the 4th century BCE, the late Achaemenid period, the inscriptions of Artaxerxes II and Artaxerxes III differ enough from the language of Darius' inscriptions to be called a "pre-Middle Persian," or "post-Old Persian". Old Persian subsequently evolved into Middle Persian, which is in turn the ancestor of New Persian.

Professor Gilbert Lazard, a famous Iranologist and the author of the book Persian Grammar, states: